Cellular composite grid-stiffened structure

ABSTRACT

A cellular composite structure includes a grid having groups of angularly intersecting ribs. The ribs of each group are oriented substantially in the same direction to each other and angularly oriented from the other rib groups. An additional rib defines a composite structure outer perimeter wall and can be differently angularly oriented from the other ribs. A contiguous rib wall is created by segments of ribs defined by rib intersections. The contiguous rib wall bounds a cavity. A multilayer sheet cap member with extending walls to engage the contiguous rib wall is positioned within the cavity. The engagement walls extend from individual sheet perimeter portions angularly oriented to the sheet. The ribs and cap member have pre-impregnated resin. Heating the cap member and ribs activates the resin and co-cures the composite structure.

FIELD OF THE INVENTION

The present invention relates in general to grid structures and morespecifically to a composite material grid-stiffened structure and methodof manufacture.

BACKGROUND OF THE INVENTION

Metallic grid-stiffened structures have been used in aerospace and otherindustry applications due to their structural efficiency as stiffenedpanels, shells and tanks. Composite grid-stiffened structures provide abetter strength-to-weight ratio for these applications resulting inlower weight and increased payload capacity. Composite grid-stiffenedstructures also provide an alternative to honeycomb-core structures.Grid stiffened structures have no moisture entrapment problems andrequire no core forming, placement and/or potting.

Current methods to fabricate composite grid-stiffened structures oftenhave resin layers between each grid stiffener and the attached skin.Current fabrication methods utilize filament winding or fiber placementto lay down tow for both the stiffeners and skin, the advantage beingthat a single, integrated co-cured structure can be fabricated usingautomated machinery. However, this results in a laminate skin that isjoined to the stiffeners with only the layer of resin forming the bondline and with no fibers crossing the bond-line.

Grid stiffeners commonly have unidirectional fiber placement. Theresulting stiffener is strongest in the directions parallel to thestiffener axis and weakest in the directions transverse to its axis,because all the fibers are generally positioned parallel to thestiffener axis. Transverse stiffener strength is therefore a structurallimitation. Further, if disbanding occurs anywhere in the stiffener/skinbond line, the disbanding can propagate throughout the structureunimpeded. The bond between the stiffeners and the body skin is notmaximized because of the lack of interweaving between the skin and theparallel positioned stiffener fibers. Current composite grid-stiffenedstructures having only a layer of resin between the stiffeners and skinare therefore best suited for expendable structures, such as rocketlaunchers, and not for structures which require long life and damageresistance such as aircraft and similar vehicles.

SUMMARY OF THE INVENTION

According to one preferred embodiment, a cellular compositegrid-stiffened structure of the present invention provides a gridincluding a plurality of intersecting ribs arranged in a plurality ofrib groups. The ribs of each group are oriented in a non-crossingpattern with respect to each other and are angularly oriented withrespect to the ribs of the other groups. A plurality of ribintersections define a plurality of rib segments. A plurality ofcontiguous rib walls are each configured as a closed geometric shape.Each of the contiguous rib walls include a selected group of the ribsegments sharing selected ones of the rib intersections to define theclosed geometric shape. Each of the contiguous walls define one of aplurality of composite structure cavities. A plurality of cap membersare each slidably receivable within a selected one of the cavities andeach is bondable to a substantial portion of a length of the contiguousrib wall defining the selected one of the cavities. Each of the capmembers is created from multiple material layers bonded together.

According to another preferred embodiment for a cellular compositestructure of the present invention, the structure includes a pluralityof first ribs substantially oriented in a first direction. A pluralityof second ribs are substantially oriented in a second direction and areangularly disposed with respect to the plurality of first ribs. Each ofthe plurality of second ribs intersect at least one of the plurality offirst ribs. A plurality of third ribs are substantially oriented in athird direction and are disposed angularly different with respect toeach of the first and second ribs. Each of the plurality of third ribsintersect at least one of the plurality of first ribs and at least oneof the plurality of second ribs. A plurality of contiguous walls areeach defined by the intersection of one each of the first, second andthird ribs. Each contiguous wall defines and bounds one of a pluralityof cavities. A plurality of cap members are each slidably receivablewithin a selected one of the cavities and are shaped to engage asubstantial length of the contiguous wall of the selected cavity. Eachcap member includes a multilayer cap sheet shaped to substantially matcha shape of the contiguous wall of the selected cavity. A plurality ofengagement walls each extend from a perimeter portion of the cap sheetand are angularly oriented with respect to the cap sheet. The engagementwalls bond with substantially all of the contiguous wall of the selectedcavity.

According to still another preferred embodiment of the presentinvention, a method is provided for creating a cellular compositestructure. The composite structure includes a plurality of intersectingribs. Portions of the intersecting ribs create a plurality of contiguouswalls each defining a cavity. The method includes shaping a sheet havingbondable layers of resin pre-impregnated cloth to substantially match aninner perimeter of one of the contiguous walls. The method furtherincludes bending a freely extending portion of each of the cloth layersto create a plurality of walls about a perimeter of the sheet. Stillfurther, the method includes positioning the sheet within one of thecavities having each of the walls of the sheet in contact with thecontiguous wall. The method also includes heating the sheet and gridstructure to adhesively bond each of the bondable layers to each otherand each of the walls to the contiguous wall.

Cellular composite structures according to the present invention provideseveral advantages. By providing separate cap members having multiplelayers of material to create each of the associated riser walls bondableto individual intersecting ribs, structural integrity of the cellularcomposite structure is increased. When the cap members are thermallybonded to the intersecting ribs of the grid, a close dimensional fit ofthe cap members ensures that the flow of resin leaves few or no gapswhere a crack can initiate and propagate from.

The features, functions, and advantages can be achieved independently invarious embodiments of the present invention or may be combined in yetother embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a perspective view of one embodiment of a cellular compositestructure according to the present invention;

FIG. 2 is a perspective view similar to FIG. 1;

FIG. 3 is a partial cross-sectional elevational view taken at Section 3of FIG. 2;

FIG. 4 is an end elevational view showing the assembly of multiplelayers of material about an elastomeric tool of the present invention;

FIG. 5 is an end elevational view similar to FIG. 4 showing the layersof material following bending about the elastomeric tool;

FIG. 6 is a perspective view showing an individual cavity formed byintersecting ribs of the present invention having an installedelastomeric tool and a cap member of the present invention;

FIG. 7 is an end elevational view showing the assembly of components forvacuum forming and heat treatment of the composite structure of thepresent invention;

FIG. 8 is a plan view of a material block identifying an exemplarycut-out pattern for a plurality of elastomeric tools of the presentinvention;

FIG. 9 is a plan view of an individual pre-impregnated cloth layer for acap member of the present invention;

FIG. 10 is a perspective view similar to FIG. 2, showing anotherpreferred embodiment of a cellular composite structure according to thepresent invention; and

FIG. 11 is a perspective view of a non-cured grid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

According to a preferred embodiment of the present invention andreferring generally to FIG. 1, a cellular composite structure 10 of thepresent invention includes a grid 11 having a plurality of intersectingribs 12. Between the intersecting ribs 12 a plurality of apertures orcavities 14 are created. The cavities 14 have a closed geometric shapeshown as a regular triangular shape, however, the invention is notlimited to triangular shaped cavities. Other shapes including but notlimited to rectangles, squares diamonds, parallelograms, hexagons andthe like can also be used. Similarly shaped aircraft structural membersare disclosed in U.S. patent application Ser. No. 10/846,861, filed May14, 2004, the disclosure of which is incorporated herein by reference.Generally, the cavities 14 are designed to have the same shape, i.e., atriangle of a common size. Some modification of cavity shape can occurwhere specific tailoring for load paths or cutouts in the structure arerequired. Each cavity 14 receives a composite cap member 16 made fromfiber reinforced resin and having a first, second and third engagementor riser wall 17, 18, and 19 along all edges. The riser walls contactthe corresponding rib segments which bound cavity 14 and define aperimeter of cavity 14. The riser walls cure, co-cure and/or adhesivelybond to the intersecting ribs 12 during final processing of cellularcomposite structure 10.

In one embodiment, each of the riser walls 17, 18, 19 are orientedsubstantially perpendicular to a cap sheet or end wall 20. Cap end wall20 is formed from a plurality of layers of material. Each layerintegrally provides one of the riser walls 17, 18, 19 respectively. Alength “L” of each riser wall 17, 18, 19 can vary depending upon thegeometric shape of the cavity 14 and spacing between intersecting ribs12. In another aspect (not shown) the riser walls of the cap members canbe non-perpendicularly oriented with respect to end wall 20.

A unitary sheet 22, provided for example of a resin pre-impregnatedcloth material or fiber/polymeric sheet can optionally be bonded to endfaces of the grid 11 as well as each of the cap end walls 20. Thepurpose of unitary sheet 22 is to provide a smooth, continuous outersurface for cellular composite structure 10. If unitary sheet 22 is notused, cellular composite structure 10 has an outer facing wall which issubstantially created by each of the plurality of cap end walls 20.

For the exemplary triangular-shaped configuration, grid 11 includes aplurality of ribs including a first rib 24, a second rib 26, and a thirdrib 28 which together form a rib group I. A first cross rib 30 and asecond cross rib 32 form a rib group II. A first alternating cross rib34 and a second alternating cross rib 35 form a rib group III. Each ofthe ribs of each individual group are oriented in substantially the samedirection with respect to each other and each of the ribs of each ribgroup are angularly oriented (oriented in a different direction or at adifferent angle) with respect to the ribs of the other rib groups.Cellular composite structure 10 can also be provided with one or moreperimeter wall ribs 36. Perimeter wall ribs 36 (only one of which isshown for clarity) is angularly oriented (oriented in a differentdirection or at a different angle) with respect to any of the ribs ofrib groups I, II or III.

In the embodiment shown, an included angle α is provided between theribs of rib group I and the ribs of rib group II. An included angle β isprovided between ribs of rib groups II and III.

As best seen in reference to FIG. 2, a plurality of cap members 16 areshown after installation within the plurality of cavities 14 and thermalbonding to riser walls 17, 18 and 19. In one preferred embodiment of theinvention, each of the ribs 12 is formed from unidirectional tows andeach of the cap members 16 are formed from one or more layers of a clothmaterial, each of the tows and the cloth layers pre-impregnated with aresin which when activated acts as an adhesive. In one aspect, thematerial of ribs 12 is a graphite/polymeric material combination havingmultiple tows of fibers running parallel to one another in a directionof a rib length. In this aspect of the invention, the material of capmembers 16 is also a graphite/polymeric material combination formed asmultiple layers of cloth material, each cloth layer having a pluralityof graphite/polymeric material fibers running in substantiallyperpendicular directions. To maximize the strength and tear resistanceof cellular composite structure 10, the orientation of the fibers ofeach cloth layer of cap members 16 are angularly offset from each otherwhen the individual layers are stacked. Other materials can be used inplace of graphite, including but not limited to fiberglass or KEVLAR®.The materials are preferably pre-impregnated with a resin such as anepoxy.

A plurality of rib intersections are created by the plurality ofintersecting ribs 12. For example, rib intersections 37, 38 and 39define a triangular shape for cavity 14 and cap member 16. Ribintersection 37 divides second cross rib 32 into two or more ribsegments including a rib segment 32′. Similarly, rib intersection 37divides second alternating cross rib 35 into two or more rib segmentsincluding rib segment 35′. Rib intersection 39 divides first rib 24 intotwo or more rib segments including rib segment 24′. A plurality ofcontiguous walls are created by selected rib segments. For example, acontiguous wall 33 is created by each of rib segments 24′, 32′ and 35′,respectively. Each riser wall 17, 18 and 19 of cap member 16 contacts acorresponding rib segment 24′, 32′ or 35′, when cap member 16 isinserted or seated in cavity 14. A plurality of similar cap members 16are similarly disposed in each of the other cavities 14 of cellularcomposite structure 10. The shape of each cap member 16 is defined bythe selected group of rib segments and rib intersections for the othercavities 14. The cap members 16 generally have a similar or equal outerperimeter and shape. An exception to this are cap members disposed incavities adjacent to the perimeter wall ribs 36. For example ribintersections 37, 38 and 40 define a different triangular-shape of anouter located cavity 14 than the interior located cavities 14.

The riser wall lengths “L” (identified for second riser wall 18 inFIG. 1) are shorter than a rib segment length “M” of the rib segments(for example, rib segment 32′). This permits seating the cap members 16within each cavity 14. A total contiguous wall length of contiguous wall33 is the sum of each group of contiguous rib segment lengths, forexample the lengths of rib segments 24′, 32′ and 35′, respectively. Theriser wall length “L” for each riser wall of a cap member 16 is shorterthan rib segment length “M”, but for bonding purposes is closely matchedto each rib segment length “M”. Therefore, the riser walls of each ofthe cap members 16 are bonded to a maximum length of each contiguouswall. This provides maximum structural integrity for cellular compositestructure 10. After assembly of cellular composite structure 10, and ifunitary sheet 22 is not used, a structure outer face 41 including eachof the cap end walls 20 of the cap members 16 provides an outer face forcellular composite structure 10.

Referring next to FIG. 3, a partial cross sectional view through anexemplary rib 42 and two cap members identifies the plurality of layersof material used to construct each cap member and each rib. A first capmember 43 and a second cap member 44 are disposed on opposite sides ofrib 42. First cap member 43 includes first, second and third layers 45,46, and 47 of a resin pre-impregnated cloth material. An integralextension of first layer 45 provides riser wall 48 which is bondable toa first side 50 of rib 42. Similarly, a riser wall 52 of second capmember 44 is provided from a first layer of material and bondable to asecond side 54 of rib 42. The total number of layers of each of firstand second cap members 43, 44 create end walls 56 and 58. The otherrisers (not shown in this view) for each of first and second cap members43, 44 are integrally provided by one of the remaining layers of capmembers 43, 44.

Rib 42 includes a stiffener end face 60. The end walls 56, 58 aresubstantially parallel to stiffener end face 60 in this embodiment. Foralternate embodiments, where cellular composite structure 10 is requiredto form a non-linear surface, for example the curving wall of a tank,end walls 56, 58 can be formed in a necessary shape to support thegeometric shape of cellular composite structure 10 and may not beparallel to end face 60.

As further shown in FIG. 3, rib 42 includes a stiffener height “A” and astiffener thickness “B”. In one embodiment, stiffener height “A” isapproximately 1.9 cm (0.75 in) and stiffener thickness “B” isapproximately 0.51 cm (0.2 inches). A riser height “C” for each of theriser walls 48, 52 in a preferred embodiment of the present invention issubstantially equal to stiffener height “A”. In still other embodimentsof the present invention riser height “C” is a predetermined percentageof stiffener height “A”. A plurality of gaps 62 can result fromlocalized bending of the riser walls of each cap member adjacent to arib prior to bonding. Gaps 62 can be minimized by resultant resin flowwhen cap members 16 and grid 11 are heated to activate thepre-impregnated resin. A wall thickness “D” of each layer of cap members16 is approximately 0.178 mm (0.007 in) in a preferred embodiment of thepresent invention. Wall thickness “D” can vary at the discretion of thedesigner. Similarly, a sheet thickness “E” of unitary sheet 22 is alsoapproximately 0.178 mm (0.007 in) but can also vary at the discretion ofthe designer.

The quantity of individual layers used to construct the cap members canvary depending upon the shape and therefore the number of riser wallsrequired. Additional layers (not shown) can also be added to cap members16 to provide additional strength and/or stiffness. Each of the materiallayers of cap member 16 include fibers (not visible in these views)which are generally cross-oriented in a 0/90° configuration. It isdesirable that the direction of the fibers of each successive layer beoriented differently as the layers are stacked, such as 45/135° in thesecond layer an so forth. As an example, a fiber orientation direction“F” is shown for third layer 47 and a fiber orientation direction “G” isshown for second layer 46. Fiber orientation “F” is to the right asviewed in FIG. 3 and fiber orientation direction “G” is angledtoward/away from the viewer as shown in FIG. 3. All fiber orientationsin rib 42 are directed toward/away from the viewer in FIG. 3.

Referring to both FIGS. 1 and 4, an exemplary method to construct acellular composite structure 10 of the present invention is as follows.The ribs 12 forming grid 11 are first constructed, but are not heated or“cured”. The individual lengths of the rib segments between ribintersections forming each of the contiguous walls 33 are predeterminedby calculation. The length “L” of each of the riser walls of the capmembers 16 is then determined based on the calculated rib segmentlengths to permit a sliding or clearance fit of the cap members 16within each cavity 14. A total length “T” in an equation: T=(n×L)provides the total perimeter length of each cap member 16, where “n”equals the quantity of rib segments defining a cavity 14.

FIG. 4 identifies an exemplary first and second layer 64, 66 for a capmember 16 prior to bending about a tool 68. According to one preferredembodiment of the present invention, tool 68 is made from an elastomericmaterial including but not limited to rubber, neoprene or siliconrubber. The elastomeric material selected should be compatible with thetemperature (approximately 250 to 350° F.) used to activate the resin toadhesively bond the material layers. The elastomeric material selectedfor tool 68 should also include a higher coefficient of thermalexpansion than the materials for the ribs 12 and cap members 16. Tool 68can include radius corners such as a radial corner 70 which permit thelayers of material to smoothly bend when forming the riser walls.

As further shown in FIG. 4, first layer 64 is applied to a face of tool68 such that an overhang area “H” representing a riser wall is provided.Second layer 66 is then positioned over first layer 64 to form a layeroverlap region “J”. Second layer 66 is also provided with a similaroverhang area “H”. A third or more layers (not shown for clarity) canalso be used. The layers in the overhang areas “H” are bent about tool68 until in this example, the overhang area “H” of first layer 64contacts a tool face 72 and the overhang area “H” of second layer 66similarly contacts a tool face 74. Each of the layers of material can bepre-aligned using an optical alignment system 76 to provide a properoverhang dimension.

According to one preferred method, a vacuum enclosure 78 is provided toassist in bending the riser wall portions of each of first and secondlayers 64, 66. Vacuum enclosure 78 is sealed to a work surface 80 uponwhich tool 68 is positioned. Vacuum equipment (not shown) is used todraw a partial vacuum within a cavity 82 such that vacuum enclosure 78bends each of first and second layers 64, 66 about tool 68 and conformsthe layers to the shape of tool 68. Each of the overhang areas “H” istherefore bent in a bending direction “K” by vacuum enclosure 78. Theinvention is not limited to the use of vacuum enclosure 78. Alternatemethods to bend the riser walls of the cap members 16 of the presentinvention can also be used such as manual bending and bending in a form.

As best seen in reference to FIG. 5, a tool/material layer assembly 84includes each of first and second layers 64, 66 (additional layers asmay be required are not shown for clarity) and tool 68. First and secondlayers 64, 66 are each disposed on a mounting surface 86 and bent abouteach of a radial corner 89 and radial corner 70, respectively to formeach of a riser wall 88 and a riser wall 90, respectively. If the riserwalls are formed by a single layer of material, expansion of tool 68could be restricted. Small gaps are therefore present at the edges ofthe layers adjacent to the radial corners. These gaps are created by theindividual riser walls extending over the sides of tool 68, and allowfor expansion of the elastomeric material of tool 68 during the curingprocess. Flow of the resin pre-impregnated in the layers during thecuring process substantially fills these gaps.

Referring now to FIG. 6, tool/material layer assembly 84 is theninserted into one of the cavities 14 formed by intersecting ribs 12 ofgrid 11. To complete the subassembly of cellular composite structure 10,a plurality of tool/material layer assemblies 84 are similarly createdand inserted into each of the cavities 14.

Referring now to FIG. 7, after each of the cavities 14 of cellularcomposite structure 10 have been filled by the plurality oftool/material layer assemblies 84, a working assembly 92 is created.Working assembly 92 in the example shown includes a rib 94, a rib 96 anda rib 98. Between each adjacent pair of ribs 94/96 and 96/98 aredisposed one of the tool/material layer assemblies 84 each having oneeach of tool 68 and cap member 16 which in the exemplary embodimentshown also provides each of riser walls 88, 90. Working assembly 92 ispositioned over a temporary mounting tool 100 such as a caul plate. Inthe embodiment shown, temporary mounting tool 100 forms a generallyhorizontal support surface, however, as previously noted the shape oftemporary mounting tool 100 can also be configured to accommodate thedesired geometric shape for cellular composite structure 10. A vacuumenclosure 102 is then positioned over working assembly 92 and sealedagainst temporary mounting tool 100. Using a vacuum device 104, via avacuum line 106 a partial vacuum is drawn within vacuum enclosure 102 tohold all of the component parts of working assembly 92 tightly together.Working assembly 92 with temporary mounting tool 100 is then positionedin an autoclave and heated to the resin activation temperature(approximately 250 to 350° F.) which activates the resin pre-impregnatedin the material of the various ribs and cap members, co-curing the ribsand the cap members. After a retention period (approximately one to twohours) the resin in each of the parts has been activated and adhesivelybonds the various cap members to each of the ribs.

During the heating process, the elastomeric material of tools 68thermally expands outwardly in each of the force directions identifiedby arrows “N” and “P” to force the various riser walls into contact withthe associated ribs. The use of an elastomeric material of a silicon orRTV type rubber such as AIRCAST 3700, made by AirTech International,Incorporated of Huntington Beach, Calif. for tools 68 provides acoefficient of thermal expansion which is greater than the coefficientof thermal expansion of the materials used for the cap members and ribsand is acceptable for use at approximately 350° F. temperatures. Withinthe autoclave, a pressure is applied to the outside of the vacuumenclosure 102, such as from a pressurized nitrogen gas, to evenlydistribute the resin. After the retention period within the autoclave,working assembly 92 is removed and allowed to cool to harden the resin.A total cycle time to heat up the assembly, soak at temperature and cooldown can require about 12 to 24 hours. When cooled, the tools 68contract and are removed. Following removal of tools 68, a configurationsimilar to FIG. 2 results.

If a unitary sheet 22 (shown in FIG. 1) is desired, it is incorporatedwith working assembly 92 prior to insertion in the autoclave. In anotherpreferred embodiment (not shown), tool/material layer assemblies 84 areinserted into cavities 14 such that cap members 16, and optional unitarysheet 22 are above tools 68. For applications requiring a high degree ofsurface smoothness, such as the aerodynamic smoothness necessary forexterior surfaces of an aircraft, flatness controlled and/or curvaturecontrolled caul plates having a machined or formed surface finish can beplaced between vacuum enclosure 102 and working assembly 92 to providethe necessary surface finish for working assembly 92. This embodimentallows, for example, a male curved tool 100 to be used to fabricate apart with an external skin.

As best seen in reference to FIG. 8, each of the tools 68 can be cutfrom a pre-cast block 108 of elastomeric material as shown. A processsuch as water lancing can be used to cut each tool 68 from block 108. Aswill be obvious, any desired shape for tool 68 can be cut from block 108which permits any desired shape of any cavity 14 to be filled using atool 68.

Referring now to FIG. 9, a triangular-shaped individual end wall layer110 of a cap member 16 is shown, similar to first and second layers64,66. End wall layer 110 includes a first bondable area 112corresponding to layer overlap region “J” shown in FIG. 4. A secondbondable area 114 corresponding to a riser wall is delineated from firstbondable area 112 by a reference bend line 116. After removal of each ofa waste area 118 and a waste area 120 the desired riser wall length “L”is provided. A plurality of end wall layers 110 can be cut from a singlelarger piece of material. Two or more end wall layers 110 are then usedto construct each of the cap members 16. As previously noted herein,each second bondable area 114 which corresponds to a single riser wallis integrally provided by only one layer of material to construct eachcap member 16. The reference bend lines of each end wall layer arealigned with the radius corners of tool 68 for bending.

Referring now to FIG. 10, a cellular composite structure 122 accordingto another preferred embodiment of the present invention includes a grid124 having a plurality of intersecting ribs which create a plurality ofcavities 126. Cellular composite structure 122 provides for generallyrectangular or parallelogram shaped cap members 128 in addition to othergeometric shapes as shown. Grid 124 includes first, second and thirdribs 130, 132, 134 respectively. First and second cross ribs 136, 138are oriented at angle α from each of the first, second and third ribs,130, 132, 134. At least one perimeter wall rib 140 is also providedsimilar to perimeter wall rib 36. The rectangular shaped cap member 128includes first, second, third and fourth riser walls 142, 144, 146, 148,respectively. Each of the riser walls 142, 144, 146, 148 are integrallyincluded with an individual material layer used to form a cap end wall150.

Referring back to FIG. 2, a crack “Y” which forms between a riser wallof the present invention and one of the ribs is isolated to theparticular rib segment where the crack forms because of the individualbond length of the associated riser wall and is therefore restrictedfrom propagating beyond the individual rib segment. Similarly, a hole ordefect “Z” occurring in a cap end wall of the present invention is alsolimited in propagation both by the different fiber orientationdirections of adjoining layers of material within the particular cap endwall and the rib segments, thereby preventing propagation betweenadjoining cap members.

Referring now to FIG. 11, exemplary intersections of the tows whichsubsequently form intersecting ribs 12 are shown prior to the curingprocess. First, second and third pre-formed ribs 152, 154 and 156 areangularly oriented with respect to each other. In this example, firstpre-formed rib 152 includes first, second and third tows 158, 160 and162. Second pre-formed rib 154 includes first and second tows 164 and166. Third pre-formed rib 156 includes first and second tows 168 and170. Each of the pre-formed ribs intersects only one of the otherpre-formed ribs in an offset manner, creating exemplary intersections172, 174 and 176. Intersections created in this offset manner maximizethe strength of grid 11 and cellular composite structure 10. Duringcure, the tows of each of the pre-formed ribs are compressed toapproximately half their non-cured width by expansion of tool(s) 68,which creates intersecting ribs 12 having approximately the same height.

Cellular composite structures according to the present invention provideseveral advantages. By providing separate cap members having multiplelayers of material to create each of the associated riser walls bondableto intersecting ribs, structural integrity of the cellular compositestructure is increased. When the cap members are thermally bonded to theintersecting ribs of the grid the close dimensional fit of the capmembers ensures that the flow of resin leaves few or no gaps where acrack can initiate and propagate from. A hole or crack which initiatesin any cap end wall is prevented from propagating to an adjacent capmember because of the multi-layer construction of the individual capmembers and the individuality of each of the riser walls used to connectthe cap members to the intersecting ribs. Similarly, if a crack formsbetween a riser wall and a rib of the present invention the crack isprevented from propagating beyond a riser wall/rib intersection locationand thereby is prevented from propagating throughout the structure.

While various preferred embodiments have been described, those skilledin the art will recognize modifications or variations which might bemade without departing from the inventive concept. The examplesillustrate the invention and are not intended to limit it. Therefore,the description and claims should be interpreted liberally with onlysuch limitation as is necessary in view of the pertinent prior art.

1. A cellular composite structure, comprising: a grid including aplurality of intersecting ribs arranged in a plurality of rib groups,the ribs of each group oriented in a non-crossing pattern with respectto each other and angularly oriented with respect to the ribs of theother groups; a plurality of rib intersections defining a plurality ofrib segments; a plurality of contiguous rib walls each configured as aclosed geometric shape, each of the contiguous rib walls having aselected group of the rib segments sharing selected ones of the ribintersections operable to define the closed geometric shape, and each ofthe contiguous walls operably defining one of a plurality of compositestructure cavities; and a cap member slidably receivable within each ofthe cavities, each cap member including at least first, second, andthird layers of material, the layers together defining an end wall, eachof the first, second, and third layers further including a riser walloriented at a predetermined angle with respect to the end wall, eachriser wall oriented parallel to and substantially in sliding contactwith an individual one of the rib segments of the selected group of therib segments; wherein the cap members and the plurality of intersectingribs are co-cured to adhesively bond the riser walls and the ribsegments to create the cellular composite structure.
 2. The structure ofclaim 1, wherein each of the ribs and the cap members comprise amaterial pre-impregnated with a thermally activated resin.
 3. Thestructure of claim 2, wherein each of the material layers of the capmembers comprise a cloth having a plurality of fibers formed of acombination including graphite and polymeric material.
 4. The structureof claim 2, wherein each rib comprises: a plurality of tows, each towhaving a plurality of fibers formed of graphite and polymeric material;wherein a directional orientation of the fibers is substantiallyparallel to a length of the rib.
 5. The structure of claim 1, whereineach cap member further comprises: the end wall having a perimeterdivisible into a plurality of perimeter portions, the perimeter operablyshaped to substantially match a shape of the contiguous rib wall; andthe plurality of riser walls together operable to contact substantiallyall of the length of the contiguous wall in the selected one of thecavities.
 6. The structure of claim 5 wherein each of the riser wallsintegrally extends from only one of the material layers.
 7. Thestructure of claim 1, further comprising a material sheet disposed overall of the end walls and operable to create a smooth and continuousouter facing surface of the composite structure.
 8. The structure ofclaim 1, wherein: the plurality of intersecting ribs include a pluralityof intersecting first, second and third ribs, each of the first, secondand third ribs angularly oriented with respect to the other ribs; andeach of the contiguous walls include one of the rib segments of one eachof the first, second and third ribs.
 9. The structure of claim 1,wherein each of the cap members comprise a triangular shape.
 10. Thestructure of claim 1, wherein the closed geometric shape comprises atriangular shape.
 11. The structure of claim 1, wherein each of thefirst, second, and third layers each have a fiber orientation, the fiberorientation of each of the layers angularly oriented with respect to thefiber orientation of the other ones of the layers.